Obstetrics Book

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Pregnancy Dating

Aka: Pregnancy Dating, Naegele's Rule, Last Menstrual Period, LMP, Estimated Due Date, Estimated Date of Confinement, EDD, EDC
  1. Definitions
    1. Actual Fetal age dated from time of conception
    2. Menstrual Age (Gestational age) = Conception + 14 days
  2. History: Naegele's Rule for calculating ED+C
    1. Start with the First Day of LMP
    2. Add 7 days
    3. Subtract 3 months
  3. History: Pregnancy history accuracy for dating
    1. In vitro fertilization: accurate to +/- 1 day
    2. Single recorded Intercourse accurate to +/- 3 days
    3. Basal Body Temp Record accurate to +/- 4 days
    4. Recorded first day of LMP accurate to +/- 2-3 weeks
    5. Quickening accurate to +/- 4-6 weeks
  4. Exam
    1. Fundal Height
      1. Indicated for pregnancy beyond 20 weeks gestation
      2. Accurate to 4-6 weeks gestation
    2. Detection Fetal Heart Tones accurate to +/- 4-6 weeks
      1. Doppler (first heard at 7-12 weeks)
      2. Fetoscope (first heard at 18-21 weeks)
    3. First trimester bimanual exam accurate to +/- 1-2 weeks
      1. Assumes normal Uterus
  5. Imaging: Obstetric Ultrasound
    1. Accurate to +/- 8% of estimated Gestational age
    2. Indications to change due date based on Ultrasound results
      1. Ultrasound at <9 weeks gestation
        1. Ultrasound due date differs from LMP-based dates by 5 days or more
      2. Ultrasound at 9 to 16 weeks gestation
        1. Ultrasound due date differs from LMP-based dates by 7 days or more
      3. Ultrasound at 16 to 22 weeks gestation
        1. Ultrasound due date differs from LMP-based dates by 10 days or more
      4. Ultrasound at 22 to 28 weeks gestation
        1. Ultrasound due date differs from LMP-based dates by 14 days or more
      5. Ultrasound at >28 weeks gestation
        1. Ultrasound due date differs from LMP-based dates by 21 days or more
    3. References
      1. (2014) Obstetric Gynecol 124(4):863-6 +PMID:25244460 [PubMed]

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